The present invention relates to an improved process for preparing crystalline mixed metal oxides, and, more particularly, to a process for preparing crystalline mixed metal oxides, such as barium titanate (BaTiO.sub.3) of narrow particle size distribution, by simultaneously mixing the respective ingredient streams continuously in a high turbulence energy environment.
Crystalline mixed metal oxides to which this invention relates belong to a class of materials typified in their simplest form by the formula: EQU ABO.sub.3
wherein,
A is a divalent alkaline earth metal ion which can be selected from, for example, Ba, Sr, Ca, Mg, Pb and mixtures thereof; and B is a quadrivalent ion which can be selected from Ti, Zr, Hf and Sn and mixtures thereof.
These materials can be prepared generally by mixing an organometallic compound or mixture of organometallic compounds of the general structure BL.sub.n, where "B" is Ti, Zr, Nb, Hf, Zn, etc., "L" is a hydrolyzable group, or a combination of such groups, such as alkoxy, aryloxy, aminoalkoxy, acetoxy, acetoacetonyl and the like, and n can be 2, 4, or 5, with aqueous or nonaqueous solutions of compounds of the general structure AX.sub.y where "A" can be selected from Ba, Sr, Ca, Mg, Pb, Nd and Li, and "X" can be selected from such groups as hydroxide, chloride, nitrate or acetate ions, and y can be 1, 2, or 3.
Crystalline mixed metal oxides are a basic material used in multi-layer ceramic capacitors. Their electrical properties depend greatly on controlling the type and amount of impurities present, particle size and particle size distribution, and stoichiometry, i.e., the mole ratio of A component to B component, of the powders from which the ceramic capacitor bodies are formed.
Numerous processing techniques have been developed for making metal oxides, e.g., solid state reactions, oxalate route, alkoxide process, in the effort to define the process parameters which can be important to control or influence the desired properties in the end product powder. Since the current trends in the capacitor industry are for thinner dielectric layers and replacement of precious metals with copper, capacitor manufacturers will require powders of uniform fine particle size (i.e., less than about 1.0 .mu.m) and powders with sintering temperatures less than the melting point of copper. In addition to a uniform fine particle size, these powders must be chemically homogeneous. The process of the present invention relates to the alkoxide processing technique, and the following prior art is relevant.
The synthesis of barium titanate by the hydrolysis of titanium alkoxides in hot aqueous barium hydroxide is reported by S. S. Flaschen, J.A.C.S. 77,6194 (1955). Tetra-n-propyl titanate (a dilute propyl alcohol solution) is added to an aqueous solution of Ba(OH).sub.2 at pH 11-14 and 80.degree.-100.degree. C. which yields BaTiO.sub.3 of 1-5 .mu.m particle size. The processing environment, e.g., the reaction pH and the concentration of barium hydroxide during the reaction, changes throughout the addition procedure so that titanium which is added to the reaction early in the procedure encounters a different reaction environment than the titanium added later in the procedure. The reaction environment is not uniform and continuous, and a barium titanate product having a uniform, homogeneous fine particle size may not always be produced therefrom.
U.S. Pat. No. 1,948,628 describes a method for preparing crystalline BaTiO.sub.3 of 1-5 .mu.m particle size by mixing an alcoholate of barium with an alcoholate of titanium, hydrolyzing with water, digesting the mixture by boiling, recovering BaTiO.sub.3 hydrate, and calcining at 1,600.degree. F. to yield BaTiO.sub.3 powder. Ball milling the calcine produces BaTiO.sub.3 having a particle size of less than 1 .mu.m.
U.S. Pat. No. 3,292,994 describes a modification of the process of Flaschen (JACS 77,6194) by which a very fine (&lt;800A) particle size barium titanate can be produced by adding gross amounts of water soluble oxygen functional compounds, such as isopropyl alcohol, methanol or ethanol, to the barium salt solution. It is well known to those skilled in the art of ceramics that such ultra fine particle size (&lt;800A), high surface area powders, e.g., having surface areas of 12.5 m.sup.2 /g and higher, cannot be satisfactorily processed into capacitors.
U.S. Pat. No. 3,647,364 describes a process for producing high purity submicron barium and strontium titanate powders by the hydrolytic decomposition of a barium and strontium alcoholate and a titanium alcoholate. The powders have a particle size in the range of 50-150 angstroms and a purity of 99.75% or higher.
U.S. Pat. No. 4,636,378 describes a low temperature method for preparing perovskite-type compounds, such as barium titanate. The process involves the sequential addition of a B-alkoxide, e.g., titanium iso-butoxide, to a predetermined amount of aqueous A-hydroxide, e.g., barium hydroxide.
None of the published art describes the preparation of mixed metal oxides having all of the following properties:
A particle size of less than 1.0 .mu.m. PA1 Particle crystallinity. PA1 Controlled cation stoichiometry. PA1 Powder surface areas less than 50 m.sup.2 /g, and preferably less than 15 m.sup.2 /g.